The present invention relates to permanent gray scale recording methods and more particularly to methods for permanently recording gray scale information in an archival memory target and to the target media produced thereby.
Modern data storage applications typically require storage of large quantities of non-erasable data. Generally, this data is stored in the digital binary format. Archival, or read-only, memory devices capable of storing large amounts of binary data, on the oder of 10.sup.15 bits per device, are compatible with modern data-processing apparatus, such as computers and the like. Storage of analog information, such as a frame of a video picture, may be accomplished by binary-digitization of the information content thereof, followed by suitable coding and storage of the coded information. However, storage in a two-valued binary memory, of a bit of information capable of assuming any one of a multiplicity of values requires that several, generally sequential, binary bits be utilized to store each binary-encoded word corresponding to one bit of multi-valued information. Thus, the amount of information storage is directly reduced by the number of binary bits required to form a single word for each multi-valued bit. It it desirable to digitize each element of an analog signal to one level of a multi-level signal, whereby each of a preselected number of "gray" levels can be threshold sensed and encoded with the single resulting gray scale bit being recorded at a single data storage location, whereby high rates of data writing (input) and reading (output) of the information stored in the system can be provided.